Tray for gas-liquid contact columns



y 1968 JEAN-MARIE LERAT ETAL 3,394,927

TRAY FOR GAS-LIQUID CONTACT COLUMNS Filed June 10, 1964 5 Sheets-Sheet 15 4 3 l Jr.

F' W I Z Fig.1?

y 30, 1968 JEANMARIE LERAT ETAL 3,394,927

TRAY FOR GAS-LIQUID CONTACT COLUMNS Filed June 10, 1964 3 Sheets-$heet 2July 30, 1968 JEAN-MARIE LERAT ETAL 3,394,927

TRAY FOR GAS-LIQUID CONTACT COLUMNS Filed June 10, 1964 3 Sheets-Sheet 50 so Tia r00 1&0 I40 1&0 1 2'00 2'20 2 10 2160 2 80 300 United StatesPatent 0 3,394,927 TRAY FOR GAS-LIQUID CONTACT COLUMNS Jean-Marie Lerat,Paris, Michel Rostaing, Orsay, and Yves Bourgeois, Vermelles, France,assignors t0 Houilieres du Bassin du Nord & du Pas-de-Calais, Douai,Nord, France, and Commissariat a lEnergie Atomique; Compagnie deConstruction Mecanique Procedes Sulzer, and lAir Liquide, Societe pourlEtude et lExploitation des Procedes Georges Claude, Paris, Seine,France Filed June 10, 1964, Ser. No. 374,198 Claims priority,application France, June 10, 1963, 937,500, Patent 1,373,686 2 Claims.(Cl. 261-114) ABSTRACT OF THE DISCLOSURE A gas-liquid contact columnhaving horizontal exchange trays which retain a liquid level. Gassupplied to the bottom of the trays travels upwardly through a verticalshaft in the tray. Distribution tubes connected to the vertical shaftdirect the flow of gas downwardly and release it in bubbles beneath theliquid surface.

Columns employing conventional trays generally comprise a verticalcylinder separated into compartments by horizontal bubble trays orhorizontal perforate plates. The liquid is supplied to the top tray viaa feed conduit and the overflow from this tray can flow into the traysituated therebeneath via a vertical discharge chute. In the case ofbubble trays, the gas rising in the column bubbles through the liquid ofeach tray since the apertures of each tray are capped by bell-shapedelements which are immersed in the liquid of that tray. In the case ofperforate trays the gas rising through the column passes through theperforations in each tray and bubbles through the liquid on the tray.

In the industrial application of isotope exchange reactions, for examplefor the production of heavy water, contact systems must be used whichenable the required isotope to pass from the gas phase to the liquidphase (or conversely) with maximum efficiency. Isotope separationrequires a large number of theoretical plates and the use of contactsystems of inadequate efiiciency would require the use of many contactsystems so that the installation would be of excessive dimensions,difficult to operate and of a prohibitive cost.

Perforate trays are frequently preferred to bubble trays because of ahigher specific rate of flow, lower pressure drop and better masstransfer efficiency than in the case of the bubble tray.

As compared with this, the bubble tray has a wider stability range andbetter liquid retention.

It has already been proposed to associate a conventional bubble traywith a perforate plate connected to the central cap of the shaft, butsuch an association does not combine the characteristics of perforatetrays with those of bubble trays.

It is an object of the present invention, therefore, to provide animproved high liquid level tray for a gasliquid contact column combiningthe characteristics of a perforate tray as regards efiiciency with theadvantage of the bubble tray as regards better liquid retention.

To this end, the tray according to the invention comprises essentially asolid plate to which the liquid phase is fed, said plate having one ormore vertical gas phase feed shafts of a relatively appreciable heightof at least 7 cm., said shaft or shafts each being connected at theirtop to a gas phase distributor system which discharges near said solidplate.

In one embodiment of the tray according to the invenice tion, the gasphsae distributor system is in the form of a cap which caps andsurrounds the vertical shaft and which is also connected at the bottomto a perforate plate disposed above the solid plate.

In a preferred embodiment of the tray according to the invention, thegas phase distributor system comprises a nest of tubes connectedradially to the top of the shaft and bent down to terminate close to thesolid plate, so that said tubes discharge near the said solid plate.

In another preferred embodiment of the tray according to the invention,the gas phase distributor system is formed by a top can disposed aroundthe shaft and continued downwardly to terminate close to the solid plateby a neat of vertical tubes open at their free ends.

According to a variant of this latter embodiment, the vertical tubes areclosed at their free ends but are perforated over a certain height oftheir side wall.

In order that the invention may be more fully understood, certainpreferred embodiments thereof will now be described, by way of exampleonly, with reference to the accompanying drawings, in which:

FIGURE 1 is a diagrammatic elevation in section showing a contact columnprovided with trays according to the invention;

FIGURE 2 is a section on the line IIII in FIG- URE 1.

FIGURES 3 to 5 are diagrammatic elevations in section of variousalternative embodiments of the invention.

FIG. 3a is a vertical sectional view of FIG. 3.

FIGURE 6 is a graph illustrating curves showing the submergence level inmillimetres of the liquid phasein this case water-against the rate offlow of the gas phasein this case airin cubic metres per hour forhigh-submergence trays according to the invention as compared withconventional reference trays of similar characteristics.

With reference to FIGURES 1 and 2 of the drawings, the column comprisesa vertical cylinder 1 which is separated into compartments by a numberof solid trays 2, to which liquid is fed via a feed chute 3 and fromwhich liquid is discharged via a discharge chute 4. Each tray comprisesone or more gas feed shafts 5 of a similar design to the conventionalbell-shaped elements of a bubble tray but of a much greater height, theheight being at least 7 cm. Each shaft is capped by a cap 6 whichsurrounds the shaft and the bottom part of which is connected to aperforate plate 7, the perforations of which have the normalcharacteristics (apertures, pitch) of a conventional perforate plate.Sealing-tightness between the tray 2 and the walls of the contact columncylinder 1 is obtained in the same way as with a conventional tray.

For a small gas flow (and for zero flow) the liquid is supported by thetray 2, the level of the liquid being regulated by the height of theshaft and of the discharge chute.

As the rate of flow of gas increases, it rises in the shaft, turns downinto the annular space between the shaft and the cap, then into thespace between the tray 2 and the plate 7, is distributed throughout theapertures of the perforate plate and, as the rate of flow increases, itexpels the liquid from the space between the tray and the perforateplate. For a given rate of flow, all the liquid is on the top perforateplate which acts exactly as a conventional perforate plate but with amuch greater height of liquid, about some tens of centimetres, as willbe seen hereinafter.

A column provided with trays as shown in FIGS. 1 and 2 was used for adeuterium isotope exchange process between liquid ammonia and ammoniasynthesis gas under the following operating conditions:

Operating pressure: 500 kg. per sq. cm.

Temperature: -30 C.

Catalyst concentration: 4% by weight (KNH Inside diameter of contactcolumn: 32 cm.

Rate of flow of gas: 2500 cubic metres per hour NTP a-F 2)- Theperforated trays included the following features:

Total section reserved for feed chute and discharge chute:

100 sq. cm.

Diameter of apertures: 0.2 cm.

Pitch: 4

Perforate area: 6% of perforable area Number of caps: 1

Free shaft section: 15.4 sq. cm. (diameter=l.4 cm.)

Free cap section: 36 sq. cm. (diameter=2.3 cm.)

Height of liquid on perforate plate: 30 cm. (6 times that of theconventional tray).

The measured efiiciency of the column was 12.5% with an estimated 85%tray obstruction and an inter-tray spacing of about 1.20 meters,corresponding to an equivalent height of a theoretical plate of 9 to 10meters. This efficiency is several times that which would be obtainedwith a conventional tray operated under similar conditions.

In the embodiment shown in FIGURE 3 of the drawings, the tray is formedof a solid plate 2 having a central gas feed shaft which is radiallycontinued at the top by a gas distributor 8 formed by a nest ofsmalldiameter tubes disposed with a regular pitch and bent down toterminate close to the tray 2. These tubes dip into the liquid on thetray where they distribute the gas and disperse it finely within theliquid phase. The liquid supply 3' to the contact system and thedischarge chute 4 are of conventional type.

It will readily be seen that it is impossible for the tray to sweat andthe liquid level retained is regulated by the height of the dischargechute and the length of the gas phase distributing tubes.

A column provided with trays as shown in FIG. 3 was used in a NH -(N +3Hexchange process under the operating conditions defined above. The gasphase distributing tubes of the trays had a diameter of 0.4 cm., thetubes had a pitch of 4 and a liquid level of 40 cm. was maintained onthe trays. Under the above conditions a basic efficiency of 12% wasmeasured under conditions corresponding to an equivalent height of atheoretical plate of 11 to 12 meters. Again this is an improvement inthe efliciency available from conventional trays.

Equivalent height of theoretical plate: approximately 11-12 m.

A variant of this embodiment of the tray according to the invention isshown in FIGURE 4 of the drawing; the gas distributor is formed by a topcap 8a disposed around the shaft 5', the lower part of the cap beingprovided with a nest of vertical tubes 8b.

A second vanient of this embodiment of the plate according to theinvention is shown in FIGURE 5, in which the tubes 8"b for distributingthe gas into the liquid phase are closed at the bottom and areperforated over a certain length of their side walls.

A column using conventional perforate trays and a column provided withtrays according to FIG. 5, were used by way of comparison in aninstallation for hydrodynamic studies operated according to the simpleairwater process and under the following operating conditions:

Inside diameter of contact column: 31 cm. Rate of flow of gas: up to 300cubic metres per hour.

The conventional perforate reference tray has the following features:

Aperture diameter: 0.2 cm.

Pitch: 4

Perforate area: about 4% of perforable area Inter-tray spacing: 120 cm.

4 The tray according to FIG. 5 and made of stainless steel, has thefollowing features:

10 gas distributor tubes of a mean diameter of 2 cm. Height of perforateportion: 5 cm.

Height of shaft (including cap): 70 cm.

Inter-tray spacing: cm.

The results of tests carried out with conventional perforate tray columnand with the column provided with trays according to FIG. 5 are shown inthe graph of FIG. 6.

Curves A to G show, in millimetres of water, the variations of pressurelosses against the rate of flow of gas (air) in cubic metres per hourfrom a starting submergence height (level of liquid (Water)) on the trayrespectively equal to 0, 30, 50, 100, 150, 200 and 300 mm. in theconventional perforate tray column. The curves H to M show inmillimetres of water the variations of the pressure losses against therate of flow of gas in cubic metres per hour from respective submergenceheights of 0, 150, 300, 400, 500 and 600 millimetres respectively in thecolumn provided with trays according to the invention.

It will be apparent from the graph that with the conventional perforatetray column instability occurs after a submergence height of 70 mm.(region bounded by the instability curve X of the conventional reference:perforate tray). After a submergence height of 300 mm. the conventionalperforate tray can no longer function irrespective of the rate of flowof the gas.

The usual area of use of conventional perforate tray columns is boundedin the graph shown in FIG. 6 by the rectangle abczl (submergence heightfrom 0 to 70, and rate of flow of gas from 0 to cubic metres per hour).

For the high-submergence plate according to the invention, thesubmergence height may be as much as about 500 mm. Also for curves A andH (dry trays), the pressure losses are very different.

However, as the height of submergence of the liquid increases, thepressure losses tend to reach the same order of magnitude, and in somecases (see curves I and G in FIG. 6), the loss of pressure of theconventional perforate tray is even higher than that of the tray whichis the object of this invention.

With regard to the exchange efficiency, the same trays as above havebeen tested in an installation under pressure for isotope exchange NI-IH It has been noticed that for the same submergence height, the exchangeefiiciency of the tray according to the invention (FIG. 5) is greaterthan that of the conventional reference perforate tray because a greateragitation of the gas bubbles prevents their coalescence and their directascent, and this increases the contact time and exchange area.

The conditions under which the tests were conducted are the following:

Operating pressure: 500 kg. per sq. cm.

Temperature: 50 C.

Catalyst concentration: 4% of KNI-I Rate of flow of gas: l,500-3,000cubic metres per hour (brought back to normal conditions).

In the case of the conventional reference perforate tray for:

a submergence height of 5 cm. of clear liquid NH 1 a tray spacing of 25cm.,

with an estimated obstruction of 85%, an exchange efficiency of 0.8% hasbeen measured, which leads to the equivalent height of 32 metres of thetheoretical plate.

In these conditions of operation, the total pressure loss of thetheoretical plate is 7.50 met-res of water.

In the case of the stainless steel high-submergence tray according tothe invention, for:

a clear liquid NH height of 30 cm., a tray spacing of 80 cm.,

an exchange efliciency of 5% has been measured, hence the equivalentheight of 16.80 metres of the theoretical plate.

The total pressure loss of 5.80 m. of water.

For a 50 cm. level of liquid, a measured efficiency of 8%, theinter-tray spacing being 1.40 metre, the equivalent height of thetheoretical plate is metres.

The total pressure loss is 6.50 m. of water.

Such a system has two advantages:

(1) The efiiciency obtained for identical perforation and pitch to thatof a conventional tray is better than the latter, as is apparent fromthe Example hereinafter.

(2) The diameter of the contact column in which such systems areinstalled can be appreciably reduced, and this is very advantageous forhigh-pressure exchanges, as in the above-cited examples.

A column having conventional trays as previously described and a columnprovided with trays as shown in FIG. 5 were compared in a NH (N 3Hexchange process under the operating conditions previously described.The improved tray had an overall diameter of cm., the liquid dischargechutes occupied an area of 100 square cm., and the tray included sevengas distribution tubes each having a mean diameter of 2 cm. The centralshaft of the column occupied 50 square cm. and the height of theperforate portion was 16 cm. When the liquid depth was 40 cm. anexchange efficiency of 13% was measured at 0, corresponding to anequivalent height of a theoretical plate of 11 meters.

What we claim is:

1. A tray designed for use in a gas liquid column, including a liquidcontainer, having a solid bottom, adapted to be mounted in substantiallyhorizontal position within said column,

means for conveying liquid into said container,

outlet means communicating with said container for maintaining aselected liquid level of at least 7 cm. in said container,

a gas feed passage having an inlet opening beneath said liquid containerand an outlet opening near the level of said outlet means, and

distribution means including a gas confining cap surrounding the outletof said gas feed passage and a plurality of tubes each having one openend communicating with said cap and a second open end terminating nearthe bottom of said container, whereby gas passing through said feedpassage will be released in said liquid near the bottom of saidcontainer.

2. A tray designed for use in a gas liquid column, in-

cluding a liquid container, having a solid bottom, adapted to he mountedin substantially horizontal position Within said column,

means for conveying liquid into said container,

outlet means communicating with said container for maintaining aselected liquid level of at least 7 cm. in said container,

a gas feed passage having an inlet opening beneath said liquid containerand an outlet opening near the level of said outlet means, and

distribution means including a gas confining cap surrounding the outletof said gas feed passage and a plurality of tubes, each having an openend communicating with said cap and a closed end terminating near thebottom of said container, each said tube having side walls perforatedover a bottom portion of said tube whereby gas passing through said feedpassage will be released in said liquid near the bottom of saidcontainer.

References Cited UNITED STATES PATENTS 140,801 7/1873 Van Syckel 261-77X 699,572 5/1902 Rocca 261114 X 1,748,855 2/1930 Teter. 1,806,090 5/1931Seguy. 1,858,158 5/1932 Laird. 2,146,651 2/1939 Prigge 261-113 2,871,0031/1959 Galbreath 202158 X 3,215,504 11/1965 Hagbarth 261114 HARRY B.THORTON, Primary Examiner.

E. H. RENNER, Assistant Examiner.

